This invention relates to the field of antennas, and more particularly to a geodesic lens antenna for use in scanning.
Scanning for radiating emitters or reflecting objects can be a difficult and time-consuming procedure. Frequently, signals are not received because they are radiated for only a very short time period and reception equipment is not responsive enough to detect such signals. A further problem arises where the receiving equipment does not have the bandwidth necessary to detect signals of widely differing frequency. Thus, considerations involved in constructing an antenna system usable to detect radiating emitters and reflecting objects include a wide scanning angle to scan as large an area as possible, a rapid scan rate to receive short duration emissions, a wide frequency range to detect as wide a range of emitters as possible, low internal losses in order to detect low level signals, constant high performance and constant beam shape over the complete scan angle in order to maintain a consistently high probability of detection over the entire scan angle. These considerations are discussed in relation to the invention in the following paragraphs.
In a radar application or in an application where the antenna is involved in only a "listening" mode, constant beam shape and constant performance over the whole scanned area is desirable in order to detect an unexpected object and to accurately map its location. There is no particular azimuth angle where best performance is preferred since unexpected objects may appear anywhere. Thus, the ability to rapidly scan a beam of constant shape over as wide an azimuth angle as possible is highly desirable.
The ability to receive and process signals over a wide frequency range is also desirable. Since the antenna is the first apparatus in the chain of received signal processing equipment, the bandwidth of the antenna can restrict the system bandwidth. Thus, an antenna with as wide a frequency range of reception as possible is desirable in order to increase the probability of detection of objects of unknown frequency. Problems in bandwidth are particularly noticeable in prior art antenna systems which use microwave circuit techniques including power dividers, couplers, hybrid devices, etc. and constrained transmission lines. In order to have a broadband antenna system each element, junction and interface must be electrically matched and must be individually broadband. As is well known to those skilled in the art, designing a broadband antenna while employing such devices and constrained transmission line can be extremely difficult due to the differing and interacting electrical properties of each element.
As stated previously, a further consideration in the detection and tracking of objects is the inherent losses of the antenna system. In order to detect low level signals, a relatively efficient and low loss antenna is required so that the signal will not be dissipated by the antenna apparatus before it reaches the remaining signal processing equipment. Prior art systems which use constrained techniques, microwave devices, junctions, and high loss dielectrics dissipate a sometimes unacceptable amount of signal due to inherent losses. Examples of such losses are insertion losses, losses due to device interactions and standing waves caused by various interfaces. Thus the designer of a low loss antenna faces many of the same problems as the designer of a wide bandwidth antenna.
In relation to scan speed, prior art systems which operate at K-band frequencies include mechanically steerable, narrow beam antennas which may be computer-controlled. Since the antenna beam is scanned by the mechanical motion of the antenna, the scan rate is relatively slow and consequently the probability of detection of a short duration signal is relatively low.
Another prior art system is the phased array antenna. The scan rate in this system is higher than the mechanical systems due to computer control and electronic steering. However, the bandwidth of a phased array system is relatively narrow and the beamwidth changes with the scan angle. In addition, the phased array system is frequency sensitive in that the beam position will shift with a frequency change. While a phased array antenna system can be used to listen to a wide angle sector without a scanning action, the bandwidth in this operational mode is even narrower than in the scanning mode. Therefore, both of these prior art systems realize relatively poor performance in wide angle listening and scanning operations.
Antennas designed on the basis of optical principles have been more successful in satisfying the requirements for a rapid scanning antenna. In an optical system, energy propagation is determined by the laws of geometrical optics and so octave bandwidths and operation in the millimeter wavelength region are more easily attainable. Propagation is in accordance with ray angles or path lengths along rays which is independent of the operating frequency. Signal dissipation is low since air filled, unconstrained transmission paths may be used. A prior art system based on optical techniques is the Rinehart antenna. This type of antenna is well known in the art for having the ability to scan theoretically perfectly.
The Rinehart antenna is a configuration type antenna structure and is specifically described in the following publication; R. F. Rinehart, A Solution of the Problem of Rapid Scanning for Radar Antennae, Journal of Applied Physics, Vol. 19, September 1948. As can be noted, Rinehart's antenna is the open waveguide analog of a variable dielectric Luneberg lens. There are two parallel conducting elements which are configured in a dome-like shape. It is thought by those skilled in the art that energy which traverses the area between the two elements follows an arithmetic mean surface between them. Thus the objective of shaping the two conducting elements is to form this arithmetic mean surface such that when energy is introduced between the two conducting elements from a point source on their periphery, energy will emerge from this structure diametrically opposite to the point source and will take the form of a collimated beam. Likewise, energy from the external environment which is in the form of a collimated beam and which strikes the Rinehart antenna will be focussed at a point on the periphery diametrically opposite the line tangent to the antenna and normal to the collimated beam.
A basic theory upon which the operation of Rinehart's antenna and other geodesic antennas are based is Fermat's least time principle; that is, electromagnetic energy is propagated along geodesics on the arithmetic mean surface which is formed between parallel conducting plates. Thus, Rinehart's antenna changes path lengths by configuring the arithmetic mean surface into a dome-like shape so that there are paths of equal length from a point on the periphery of the antenna to all points on a line tangent to the periphery and located diametrically opposite the point. The Rinehart antenna has theoretically perfect scanning properties, however, the direction of flow at the periphery is parallel to the central axis about which the dome-like elements are revolved. The desired direction of flow is in the plane normal to the axis such that a wide area may be scanned. Thus, an efficient reflector or lip is required at the periphery which will direct the energy but which will not create prohibitively large reflections or defocus that energy. A method to achieve this result is found in U.S. Pat. No. 2,814,037 entitled "Scan Antenna" to Warren et al.
The Warren et al. patent concerns a modification of the Rinehart antenna. This modification purportedly directs the energy at an angle to the central axis, in an outward direction. In order to retain the theoretically perfect focussing property in the scan plane in accordance with the Rinehart theory, Warren et al. has reshaped the geodesic dome to accommodate the lip that was added. The resulting antenna has a narrow beam in azimuth which is scanable over a wide azimuth angle, however, there is a relatively broad beam in elevation. The terms azimuth and elevation are used herein in accordance with their meanings as are well defined in the art, azimuth refers to angular position in a horizontal plane and elevation refers to angular position in a vertical plane. However, it is to be understood that the terms are relative and are merely used to establish reference planes in order to make visualization of antenna operation somewhat easier.
A broad beam width in elevation is an undesirable property in certain applications. For example, in many object detection and tracking applications, a narrow to moderate beamwidth in both azimuth and elevation is desirable. This narrower beamwidth has beneficial effects, one of which is the capability to scan a greater distance due to energy concentration. Prior art geodesic antennas disclose a means of focusing or compressing the beam in elevation through the use of parabolic reflectors, reflector feed assemblies, and parabolic-cylinder reflectors. An example of such an apparatus is found in U.S. Pat. No. 3,343,171 entitled "Geodesic Lens Scanning Antenna" to Goodman.
The Goodman patent purportedly achieves a compressed vertical beamwidth through the use of reflectors. However, several substantial disadvantages exist with this method of achieving vertical directivity. The first is that the reflecting apparatus required is commonly larger than the geodesic antenna dome thereby making the total antenna apparatus a large mass and subject to various physical interferences such as wind impact. Secondly, there is poor aperture efficiency due to the relatively large size of the reflector and the fact that the entire reflector is not illuminated for all beams. Thirdly, the apparatus is not circularly symmetrical due to the use of a reflector therefore the beamwidth will change with scan angle and several reflectors will be required for large azimuthal coverage.
Thus, even though antenna systems based upon optical principles exist in prior art, the deficiencies of these prior art systems result in relatively poor performance in wide angle scanning or listening applications.